This invention relates to spinel manufacture. More specifically, it relates to an improved process for the production of high purity magnesium aluminate spinel.
Pure stoichiometric magnesium aluminate spinel has been produced on a small scale by electric fusion processes and has demonstrated its utility as a superior refractory material. The following comparison of the spinel with other common refractories indicates why: Refractory Spinel Alumina Mullite Periclase __________________________________________________________________________ Chemical Composition MgAl.sub.2 O.sub.4 Al.sub.2 O.sub.3 3Al.sub.2 O.sub.3.2SiO.sub.2 MgO Properties Melting point, .degree.C. 2135 2015 1830 2800 Density, g/cc 3.6 4.0 3.2 3.6 Mean specific heat, cal/gm .degree.C. (20-1000 .degree.C.) 0.257 0.270 0.238 0.287 Mean reversible thermal expansion cm/cm .degree.C. 7.6.times.10.sup.-.sup.6 8.8.times.10.sup.-.sup.6 5.3.times.10.sup.-.sup.6 13.5.times.10.sup.-.sup.6 (20-1000 .degree.C.) Thermal conductivity, cal/cm.sup.2 sec (.degree.C./cm) 100.degree.C. 0.036 0.072 0.014 0.090 1000.degree.C. 0.014 0.015 0.009 0.017 __________________________________________________________________________
In addition, the spinel exhibits first deformation at 2 kg/cm.sup.2 at 2000.degree.C.; does not react with silica until 1735.degree.C., with magnesia or calcium oxide until they are taken into solid solution at 2000.degree.C. or with .alpha.-alumina until 1925.degree.C.; can be used to hold all metals except alkaline earths; and shows better resistance to alkalis than alumina, better spall resistance than chromemagnesite refractories and excellent basic slag resistance.
The many attempts to produce a low cost magnesium aluminate spinel grain of high purity have not resulted in commercial success. These attempts typically involved the following steps: physical mixing of high purity sources of alumina and magnesia; calcination at 900.degree. to 1500.degree.C.; grinding; pelletizing and finally sintering at 1600.degree. to 1900.degree.C. The technical complexities of such processes are responsible for the present very limited use of magnesium aluminate spinel as a refractory.